Datasheet

PCF2127AT All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 6 — 11 July 2013 27 of 86
NXP Semiconductors
PCF2127AT
Integrated RTC, TCXO and quartz crystal
8.7 Oscillator stop detection function
The PCF2127AT has an on-chip oscillator detection circuit which monitors the status of
the oscillation: whenever the oscillation stops, a reset occurs and the oscillator stop flag
OSF (in register Seconds) is set logic 1.
• Power-on:
a. The oscillator is not running, the chip is in reset (pin RST
is LOW and flag OSF is
logic 1).
b. When the oscillator starts running and is stable after power-on, the chip exits from
reset (pin RST
is HIGH).
c. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) by command.
• Power supply failure:
a. When the power supply of the chip (V
BBS
, see Section 8.6.2) drops below a certain
value (V
low
), typically 1.2 V, the oscillator stops running and a reset occurs.
b. When the power supply returns to normal operation, the oscillator starts running
again, the chip exits from reset.
c. The flag OSF is still logic 1 and can be cleared (OSF set logic 0) by command.
(1) Theoretical state of the signals since there is no power.
(2) The oscillator stop flag (OSF), set logic 1, indicates that the oscillation has stopped and a reset has
occurred since the flag was last cleared (OSF set logic 0). In this case, the integrity of the clock
information is not guaranteed. The OSF flag is cleared by command.
Fig 13. Power failure event due to battery discharge: reset occurs
001aaj409
internal power supply
(1)
V
th(sw)bat
(= 2.5 V)
V
SS
V
BAT
V
low
(= 1.2 V)
V
DD
V
BBS
V
BBS
V
BBS
V
BBS
V
DD
V
BAT
V
SS
(2)
OSF
battery discharge